1. Field of the Disclosure
Disclosed is a package to house a Micro-Electro-Mechanical Systems device, and more particularly to a method for manufacture of a top port package and the so manufactured package.
2. Description of the Related Art
One Micro-Electro-Mechanical Systems (MEMS) device is an acoustic transducer used in a microphone chip for a personal electronic device (PED). These devices include, but are not limited to, cell phones, laptop computers, tablets and mobile digital devices. One acoustic transducer is disclosed in U.S. Pat. No. 5,870,482 to Loeppert et al., titled “Miniature Silicon Condenser Microphone.” The acoustic transducer is disclosed as having a frame with a silicon nitride diaphragm bonded to one side of the frame and extending cantilever style over the other side. A gap between the frame and the diaphragm forms a variable air gap capacitor. Deflection of the diaphragm by acoustic (sound) waves changes the gap spacing creating a measurable change in capacitance. U.S. 5,870,482 is incorporated by reference herein in its entirety.
Demands on a package housing a MEMS device are stringent. The device must be protected from dirt, dust and mechanical damage. Electrical interconnection is required to transmit electrical signals between the device and other electrical components of the PED. Deflection of the diaphragm is against a fixed volume of air, requiring an air tight cavity adjacent one side of the device. Further, as cell phones and other electrical devices become smaller and more light weight, the package housing the MEMS device must contribute to those objectives. Likewise, cost constraints on the product require the package to be inexpensive and preferably not overly complex to assemble.
Representative packages for housing MEMS devices are disclosed in U.S. Pat. No. 8,030,722 to Bolognia et al., titled “Reversible Top/Bottom MEMS Package” and in published United States Patent Application Publication No. 2011/0075875 by Wu et al. that is titled “MEMS Microphone Package.” Both U.S. Pat. Nos. 8,030,722 and U.S. 2011/0075875 are incorporated by reference herein in their entireties.
U.S. Pat. No. 8,030,722 discloses a package for a MEMS device having a base and a cover formed from a printed circuit board material. Conductive sidewalls electrically interconnect the base and the cover. A MEMS device is mounted to either the base or the cover and wire bonds extending from the MEMS device contact circuit traces on both the cover and the base; electrically interconnecting the MEMS device to both.
U.S. 2011/0075875 discloses a MEMS package having a MEMS device mounted to the package cover. The MEMS device is electrically interconnected to circuit traces that extend from the cover to conductive sidewalls to circuit traces formed in the base. An integrated circuit device, such as an amplifier or noise filter is electrically interconnected to the circuit traces formed in the base.
Top and bottom port packages exist mainly due to the mechanical requirements of the end application. Top port packages are often preferred as bottom port packages require a corresponding hole in the application PCB to which the microphone is mounted. However, if the ratio of air behind the membrane is larger than that in front of it the package has improved technical performance particularly regarding signal to noise ratio (larger back volume of air than front volume of air). With a bottom port package where the MEMS die is connected directly to the package substrate it is easy to get this improved ratio of back to front volume by mounting the MEMS die over the sound port. For a top port package which many applications mechanically require, it is not possible with standard packaging to mount the MEMS die in such a way to get the improved technical performance.
There are times when it is desirable to mount the MEMS device to the cover of the package encasing the device. However, forming circuit traces in the cover and electrically conductive sidewalls to transfer electric signals to and from the MEMS device, as in the disclosures referenced above, results in a complex package requiring accurate alignment which runs contrary to the objectives to simplify and reduce the cost of such a package. There remains a need for a robust package that enables mounting a MEMS device to a cover component of the package that does not have the disadvantages of complex assembly and high cost.